1. Field of the Invention
The present invention relates to a brazing fin material for heat exchangers made of an aluminum alloy. More particularly, the present invention relates a brazing fin material for heat exchangers made of an aluminum alloy manufactured by joining a fin material and a working fluid passage material by brazing, such as a radiator, a heater core, an oil cooler, an intercooler, and a condenser or an evaporator for a car air conditioner, particularly to a brazing fin material for heat exchangers which rarely produces die wear when forming a fin and exhibits excellent intergranular corrosion resistance and joinability, a heat exchanger including the brazing fin material, and a method of manufacturing the same.
2. Description of Related Art
A heat exchanger made of an aluminum alloy is widely used as an automotive heat exchanger such as a radiator, a heater core, an oil cooler, an intercooler, and an evaporator and a condenser for a car air conditioner. A heat exchanger made of an aluminum alloy is manufactured by combining an aluminum alloy fin material or a brazing fin material in which a filler metal is clad on each side of an aluminum alloy with an extruded flat tube (working fluid passage material) made of an Al—Cu alloy, an Al—Mn alloy, an Al—Mn—Cu alloy, or the like, or with a tube obtained by forming a brazing sheet in which a filler metal is clad on one side of the above alloy in a flat shape, and joining the fin material and the tube through the filler metal by flux brazing using a chloride flux, inert gas atmosphere brazing using a fluoride flux, or vacuum brazing.
The filler metal is disposed on one side of the working fluid passage material or  each side of the fin material. As the filler metal, a filler metal made of an aluminum alloy containing silicon is generally used.
When using a brazing fin material for heat exchangers in which a filler metal is clad on a core material, the filler metal clad on the brazing fin material is melted by heating for brazing and flows toward the brazing joint portion to effect brazing. Silicon in the filler metal is diffused into the core material during heating for brazing. Since silicon tends to be diffused into the grain boundaries of the core material rather than into the grains, a region with a low potential is produced near the grain boundaries, whereby intergranular corrosion tends to occur in the core material after brazing.
The strength of the heat exchanger core is decreased by intergranular corrosion of the core material. When the filler metal is diffused only in the surface region of the core material, intergranular corrosion does not reach the center of the core material. Therefore, the strength of the heat exchanger core is ensured when the brazing fin material for heat exchangers has a large thickness.
In recent years, a further improvement in fuel consumption of automobiles has been demanded from the environmental point of view, and a reduction in weight has been increasingly demanded for automotive heat exchangers. Therefore, the constituent members of the heat exchanger such as a fin material and a working fluid passage material (tube material) have been reduced in thickness, and a fin material with a small thickness has been in demand.
If the thickness of the fin material is reduced, the filler metal is diffused into the core material along the thickness direction during brazing. As a result, intergranular corrosion occurs in the core material along the thickness direction after brazing, whereby the strength of the heat exchanger core cannot be ensured.
In order to solve the above problem, JP-A-2004-84060 attempts to reduce intergranular corrosion of a brazing fin material for heat exchangers by providing the core material before brazing with a fibrous structure, adjusting the grain size of the  structure after brazing to 50 to 250 μm, and specifying the metal elements and their content in the core material and the filler metal, for example.
However, when the thickness of the brazing fin material for heat exchangers is reduced to as small as 0.06 mm or less, the brazing fin material disclosed in JP-A-2004-84060 cannot sufficiently prevent intergranular corrosion.
In order to improve the heat transfer efficiency of the brazing fin material for heat exchangers with a cooling medium such as air, the coil of the brazing fin material is slit and corrugated using a forming die to increase the surface area or produce turbulent flows, thereby increasing heat exchange performance. However, when forming the brazing fin material for heat exchangers using a forming die, the die wears to a large extent in comparison with the case of forming a fin material on which a filler metal is not clad, whereby the life of the die is reduced.
When the thickness of the brazing fin material for heat exchangers is reduced, the amount of filler metal flowing toward the joint portion during brazing decreases, whereby the joinability of the joint portion deteriorates, or deformation (high-temperature buckling) due to high-temperature heating tends to occur. Therefore, the brazing fin material for heat exchangers is required to exhibit excellent joinability of the joint portion and excellent high-temperature buckling resistance, even if the thickness of the brazing fin material is reduced.
The brazing fin material for heat exchangers also has a problem in which intergranular corrosion of the core material cannot be sufficiently prevented when the thickness of the brazing fin material is reduced to as small as 0.06 mm or less, whereby it is difficult to ensure the strength of the heat exchanger core.
Accordingly, an object of the present invention is to provide a brazing fin material for heat exchangers of which the core material rarely undergoes intergranular corrosion even if the thickness of the brazing fin material is reduced to 0.06 mm or less, ensures excellent joinability of the joint portion, exhibits excellent high-temperature  buckling resistance, and rarely causes die wear during corrugating, and a heat exchanger including a fin material in which intergranular corrosion rarely occurs.
JP-A-2003-39194 discloses a brazing sheet in which the coarse silicon particles in the filler metal have a maximum diameter of 20 μm or less. However, since JP-A-2003-39194 aims at preventing formation of holes in the brazing sheet due to melting during brazing, JP-A-2003-39194 differs in object from the present invention aiming at preventing intergranular corrosion of the brazing fin material after brazing. Since JP-A-2003-39194 relates to an invention of a brazing sheet, JP-A-2003-39194 does not take corrugating into consideration.